Stocks kicked off the week with a thud, with the Dow and S&P 500 logging their first four-day losing streaks this year, as Treasury yields hit a two-year high amid ongoing worries over when the Federal Reserve might start to reduce its asset purchases.

U.S. stock index futures were narrowly mixed Monday, after major indexes posted their first back-to-back weekly losses since late June, amid ongoing uncertainty about when the Federal Reserve might start tapering its asset purchases.